This invention relates to the method for low-cost manufacture of a physical topographic pattern and more particularly to the manufacture of stretch-tunable micro and nano scale sinusoidal periodic wrinkle patterns that are generated upon compression of supported thin films.
Stretch-tenability extends the functionality of micro and nano structures by enabling the design and fabrication of active and adaptive systems that can respond to a variety of stimuli such as touch, temperature, humidity, and mechanical strain. Such active micro/nano-enabled systems have the potential to significantly impact diverse fields with direct societal benefits such as energy, water, health, and environment among others. For example, stretch-tunable structures find applications in the field of stretchable electronics, tunable optics, micro-nano fluidics, and sensing. Wrinkling of thin films is a low-cost process for fabricating such stretch-tunable structures over large areas.
Sinusoidal periodic wrinkled patterns are formed via compression of supported thin films as a result of buckling-based instabilities and the mechanism is similar to Euler buckling of beams under compressive loads. A schematic of this process is illustrated in FIG. 1. Essential elements of a system that demonstrates wrinkle formation are: (i) a film 10 that is thin relative to the base, (ii) mismatch in the elastic moduli of the film and the base 12 with the film being stiffer than the base, and (iii) loading conditions that generate in-plane compressive strain (c) in the film. In such bilayer systems, the state of pure compression becomes unstable beyond a critical strain and wrinkles 14 are formed via periodic bending of the film/base. The natural period of wrinkles (λn) is determined by the competing dependence of strain energy on period in the film versus in the base. For small strains, the natural period depends only on the thin film thickness and the ratio of Young's moduli of the film and the base. At large strains, the natural period can be tuned to some extent by the strain; in addition, the amplitude (A) is determined by amount of applied compressive strain. Thus, stretch-tunable micro/nano structures can be fabricated via wrinkling.
As the amplitude and the period of the wrinkles depend on strain, it is necessary to increase the feasible range of strain when high stretch-tenability is desired. The feasible range of strain is limited by the phenomenon of period doubling that occurs at high strains (FIG. 2). When the compressive strain exceeds the nominal onset strain (ε2,0), an additional period-doubled mode 20 emerges so that the single period sinusoidal structure transitions into a two-period structure. Emergence of this complex structure at high strains is often undesirable in applications that rely on a single-period structure. Thus, there is a need to suppress the onset of period doubling during fabrication of wrinkled structures.
In the past, the strain dependence of stiffness modulus has been used to control the onset of period doubling1,2. In that method, period doubling is suppressed by increasing the amount of pre-stretch in the system. Such a method is limited by material selection as it relies on 2nd order nonlinear effects that arise from the dependence of material properties on strain. To overcome this material based limitation, one requires a suppression technique that does not rely on the stiffness versus strain material behavior. Herein, such a suppression technique is disclosed; this technique is based on geometric modifications to the system that can be applied either separately or in combination with the pre-stretch based method. Specifically, suppression of period doubling is achieved by performing the wrinkling process on pre-patterned surfaces instead of flat surfaces (FIG. 3). In this disclosed method, the second critical strain for onset of period doubling (ε2,p) is controlled by changing the amplitude (Ap) of the pre-patterns 30. With this technique, the operating range of stretch-tunable wrinkle-based devices can be increased by a factor of at least 1.5 with a modest pre-pattern aspect ratio of 0.15 (i.e., 2Ap/λn=0.15).